Today, environmentally biodegradable polymers and biodegradable molded articles made of such polymers have been demanded from the viewpoint of environmental conservation. Active studies have been directed to aliphatic polyesters as biodegradable polymers. Lactic acid polymers, in particular, have a sufficiently high melting point, which ranges 140° to 180° C., and exhibit excellent transparency and are therefore highly expected for use as a packaging material, a transparent molded article, etc.
Containers obtained by injection molding, for example, of lactic acid polymers are rigid enough but poor in heat resistance and, in some cases, poor in both heat resistance and impact resistance. Lactic acid polymer molded articles such as packaging containers therefore have limited application because of lack of safety against hot water or microwaving.
In order to obtain heat-resistant lactic acid polymer molded articles, it has been necessary to take a long time for mold cooling or anneal the molded articles thereby to let the resin highly crystallize. However, prolonged cooling is impractical and tends to result in insufficient crystallization, and annealing after molding is liable to deform molded articles during crystallization.
It is generally known that crystallization of a resin can be accelerated by addition of a nucleating agent. For example, JP-A-60-86156 teaches that crystallization of polyethylene terephthalate is accelerated by adding fine powder of a totally aromatic polyester composed mainly of terephthalic acid and resorcin units as a nucleating agent.
Addition of additives including a nucleating agent to a biodegradable polymer is described in JP-A-5-70696, JP-T-5-504731 (WO 90/001521), U.S. Pat. No. 5,180,765, JP-T-6-504799, JP-A-8-193165, JP-A-4-220456, JP-T-7-504939, JP-A-9-278991, JP-A-10-87975, and JP-A-11-5849.
JP-A-5-70696 supra discloses a material of plastic containers that comprises a biodegradable polymer, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate), polycaprolactone or polylactic acid, and 10 to 40% by weight of calcium carbonate or talc having an average particle size of 20 μm or smaller. This technique aims to accelerate biodegradation of biodegradable plastic waste by addition of a large amount of the inorganic filler but not to improve heat resistance of a biodegradable polymer molded article by crystallization.
JP-T-5-504731 (WO 90/001521) supra teaches that hardness, strength and temperature resistance of thermoplastic polylactides can be varied by addition of an inorganic filler such as silica or kaolinite. In a working example given in the publication, an L-lactide/DL-lactide copolymer was blended with 5 wt % calcium lactate on a heated mill roll at 170° C. for 5 minutes to obtain a sheet. The sheet was stiff, strong, and hazy and revealed a substantial increase in crystallinity.
U.S. Pat. No. 5,180,765 supra describes that lactic acid or a lactic acid oligomer is useful as a plasticizer of polylactic acid, serving to reduce the glass transition temperature and impart flexibility.
JP-T-6-504799 supra recites lactates and benzoates as a nucleating agent to be compounded into a biodegradable composition containing polylactic acid. In working examples of the publication, a polylactide copolymer compound containing 1% calcium lactate was injection molded in a mold kept at about 85° C. for a retention time of 2 minutes and, because of insufficient crystallinity, subjected to annealing at about. 110° to 135° C. before removal from the mold.
In fact, JP-A-8-193165 supra mentions as follows. When the inventors tried to injection mold a lactic acid polymer blended with a commonly employed nucleating agent, such as talc, silica or calcium lactate, the crystallization was so slow, and the resulting molded article was too brittle for practical use. The inventors conclude that a lactic acid polymer compound containing such a common additive is slow in crystallization in general molding, such as injection molding, blow molding or compression molding, only to provide a molded article with a low service heat resistance (100° C. or lower) and insufficient impact resistance, and is therefore limited in applicability.
JP-A-4-220456 supra describes that addition of polyglycolic acid and/or a derivative thereof to poly(L-lactide) as a nucleating agent increases the rate of crystallization thereby to shorten the injection molding cycle time and to give molded articles with improved mechanical properties. It is mentioned that the crystallinity achieved in injection molding followed by 60 second cooling was 22.6% without the aid of the nucleating agent but increased to 45.5% with the aid of the nucleating agent. However, JP-A-8-193165 supra reports that a lactic acid polymer containing no nucleating agent failed to be injection molded under such a mold temperature condition as referred to in JP-A-4-220456, i.e., at a mold temperature not lower than the glass transition temperature.
JP-T-7-504939 supra proposes adding to a polylactide mixture an effective amount of a stabilizer for suppressing depolymerization at or above the glass transition temperature. Such a stabilizer includes antioxidants, dehydrators, desiccants, and catalyst deactivators. The proposed catalyst deactivators include alkyl hydrazines, aryl hydrazines, amides, cyclic amides, hydrazones, carboxylic acid hydrazides, bisacylated hydrazine derivatives, and heterocyclic compounds, with bis[3-(3,5-di-t-but-yl-4-hydroxyphenyl)propionic acid] hydrazide being mentioned as a preferred one. According to this technique, it is true that depolymerization is inhibited in a molten state. However, it turned out that addition of bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionic acid] hydrazide failed to provide a resin composition that could be molded under conditions employed for general-purpose resins to produce molded articles with high crystallinity and excellent transparency. In short, catalyst deactivation made no contribution to reduction in molding cycle time.
JP-A-9-278991, JP-A-10-87975, and JP-A-11-5849 supra propose adding an aromatic or aliphatic carboxylic acid amide compound to a polylactic acid or an aliphatic polyester to obtain molded articles excellent in crystallinity, transparency, and heat resistance. Nevertheless, actual experimentation revealed that the resin composition could not be molded by injection molding, etc. under cycle conditions applied to general-purpose resins, proving it difficult to put the technique into practice.
JP-A-10-158370 describes that a phenol antioxidant, such as 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate, is effective in stabilizing a polylactic acid.